Add text to an equirectangular photo
First setup your equirectangular image as an environment texture to the world surface :


Go to the world panel, in the surface part: choose 'environment texture', then your image and select 'equirectangular'.


(below, the world panel setting on the right, the node equivalent on bottom and the view rendered on top with default cube in place)

 


Add a camera, set it to 'panoramic' in its property panel, then choose 'equirectangular'. This camera will be used to render the scene, so you can here choose its orientation in order to have the same centered image as your original (as you need).





Add a text... somewhere... depending on where you want it to be. 


To adjust its position, go in camera view Numpad0 in rendered mode, select your text, and move/rotate/scale it as needed 




Now we need to render the image :


Go to the render panel, choose the size you need and the format you want, and click on 'render' :




Save your image F3 and in the options which are at the bottom left, choose the export format you want :





You are done. To test the result, simply set this new image as environment texture (as we did for the original image at the begining)